Ozone, O.sub.3, is a powerful oxidant and has many uses as a purifying agent, germicide, bactericide, and decontaminant of liquids, gases, and solids. Common uses of ozone include drinking water purification, waste water purification, pool and spa water purification, air freshening, bleaching, treatment of industrial waste, and decontamination of food.
Alternative oxidizing agents, which include dioxides, chlorine, bromine, and halogen compounds, are generally less effective than ozone. For example, ozone purifies water and air very quickly and efficiently, and thousands of times faster than chlorine. Other oxidants may also pose dangers during periods of acute exposure. For instance, they may produce a variety of illnesses or syndromes and some of their chemical by-products such as trihalomethanes or THMs and Trichlorethylene (TCE) are highly suspected to be carcinogenic substances. In addition, most of these alternative oxidizing agents require potentially hazardous steps of manufacture, transportation, and storage. Chlorine oxidation also results in the undesirable by-product hydrochloric acid, as well as additional salts in water applications. On the other hand, ozone, in the quantities needed for water purification for example, has no noticeable odour, taste or colour and is not irritating to humans or equipment. Furthermore, the only by-products generally left by ozone upon breakdown are pure oxygen and, in some cases, carbon dioxide or other harmless substances.
Ozone is generated on-site at controlled levels, and so it does not require storage or transportation since it is produced on site wherever required. Indeed, because of the short life of the O.sub.3 molecule--it has a half-life of only about 20 minutes in air (and about 2 hours in water) which makes it a powerful oxidizer--ozone cannot be stored for practical purposes. This is also an advantage, as ozone rapidly decays away to safe levels.
Ozone oxidation provides additional benefits as well. Ozone controls microbiological growth, reduces corrosion levels compared to other chemical oxidants (extending equipment life and saving in equipment replacement costs), results in no chemical residual or build up, and allows reuse of water for many applications when combined with a filtration process.
Ozone is generally formed by the action of oxygen atoms (O.sub.1) on oxygen molecules (O.sub.2). The splitting of the oxygen molecule can be achieved by applying electrical, optical, chemical, or thermal energy to the oxygen molecules. While all of these methods have been used to create ozone in the prior art, optical, chemical, and thermal energy methods are generally very elaborate and costly, and so are usually not suitable for most applications. Most optical methods such as those disclosed by Pincon in U.S. Pat. No. 4,124,467 and by Beitzel in U.S. Pat. No. 4,189,363, use ultraviolet radiation. Generally, in these systems, a special lamp inside an ozone chamber radiates ultraviolet light of a specific wavelength which converts Oxygen molecules (O.sub.2) into active oxygen (O.sub.1) and ozone (O.sub.3) molecules. However, ultraviolet radiation methods produce ozone in only minimal concentrations, which are in many cases insufficient for a particular application. In addition, ultraviolet UV lamps are bulky, so that an ultraviolet ozone generator cannot be very compact, and the lamps are also fragile, expensive, and subject to burning out.
As a result, electrical means for producing ozone, in particular by way of a corona discharge to create the ozone molecule, are the most practical and popular methods currently in use. In known manner, ozone may be produced by passing an oxygen (O.sub.2) containing gas through a high voltage discharge or corona. A corona discharge is a discharge of electricity within the gaseous dielectric (for example air) along the surface of and between the conductors or electrodes. The structure of a corona discharge device, two conductors separated by a dielectric (gas) is similar to that of a capacitor. The corona discharge, which appears as a blueish-purple glow, is due to the ionization of the insulating gas between the electrodes. This discharge occurs when the field potential gradient of an alternating current exceeds the corona start or threshold voltage and continues until the voltage is reduced or stopped. The field potential gradient is the voltage per unit length along the conductive path of the device.
For producing ozone, an oxygen containing gas is supplied as the insulating gas between the conductors (i.e. into the corona field) This causes some of the oxygen (O.sub.2) molecule bonds to split, freeing two ionized oxygen atoms which create ozone when colliding with other oxygen molecules and/or with other ionized oxygen atoms. It is usually necessary for the oxygen containing gas to be dried, and it is also advantageous for the gas to be cooled. The most readily available gas for this purpose is air which contains approximately 78% diatomic nitrogen (N.sub.2) and 21% oxygen (O.sub.2). However, a more concentrated source of oxygen can also be used. The ozone/gas mixture discharged from prior art corona discharge ozone generators normally contains from 1% to 3% ozone (by weight) when using dry air, and 3% to 6% ozone (by weight) when using high purity oxygen as the feed gas.
A solid dielectric material, in addition to the gaseous dielectric, is usually included between the two electrodes or conductors to prevent shorting between the electrodes and to intensify the electric field. The solid dielectric may be plastic, ceramic, or glass for example. The solid dielectric is gapped or spaced from at least one of the electrodes to allow for space for the air or feed gas to flow through the corona region. For structural simplicity, in many prior art devices the solid dielectric abuts or runs along one of the conductors while being spaced from the other.
One drawback of corona discharge units is that when air is used as the insulating gas, there is a potential danger that some of the high composition of nitrogen (N.sub.2) may ionize and break into singlets which react to form nitric oxide (NO), nitrogen dioxide (NO.sub.2) or nitrous oxide (N.sub.2 O). Formation of these compounds inhibits the ozone generation process. This potential problem is heightened when there is a substantial amount of water vapour in the air and when the applied electrical voltage is very high.
In general, prior art corona discharge generators are based on designs which use either flat plate conductors or circular tube conductors. The first of these designs employs relatively large surface area plates consisting of electrodes and a dielectric assembled in parallel to one another, with suitable spacing between the electrode layers to establish the arc or insulating gap. The feed gas or air is then blown in parallel across the surface of the dielectric and electrodes. The second type of prior art corona discharge design uses concentric dielectric tubes and cylindrical conductors. The dielectric lies concentrically between the two conductors, often abutting against the outer surface of the inner conductor with the outer electrode formed by a steel tube or steel mesh concentrically spaced around the outer circumference of the dielectric tube. As a result, at least the outer electrode will have a relatively large surface area. The feed gas is directed parallel to the axis of the tubes between the outer wall of the dielectric tube and the inner wall of the outside electrode tube. As high voltage AC power is applied between the concentric electrodes, corona will develop in the region between the electrodes.
These prior art designs are subject to several shortcomings. First they require a high operating voltage to sustain a corona throughout the entire insulating or dielectric area. Second, the flat or tubular electrodes, particularly mesh electrodes, and the solid dielectrics tend to sag or warp with heat resulting in random corona fields throughout the insulating region. Third, in many of these designs, only a very low percentage of feed (or oxygen containing) gas passes through the corona discharge field. Fourth, because the electrodes and the corona discharge region are relatively lengthy in many prior art designs, some of the ozone generated may decompose, under the continued presence of the corona field, before exiting the device. Fifth, because of the high operating voltages, there is a greater probability of undesirable nitrous oxide compounds being produced, as mentioned above. Sixth, these designs problematically facilitate the depositing of undesirable compounds which are generated on the electrode and solid dielectric surfaces. Seventh, these prior art designs are generally very bulky and often require multiple layers and components. Finally, prior art corona discharge devices offer little ability to control or vary the amount of ozone produced.